This invention relates generally to electric power converters comprising two or more parallel pairs of series-connected, alternately conducting, high-speed, solid-state, unidirectional electric valves of the kind that can be switched between a non-conducting state (off) and a conducting state (on) in response to appropriate turn-on and turn-off signals being alternately applied to a control electrode of the valve, and it relates more particularly to means for detecting and responding to a malfunctioning valve that fails to remain off throughout the interval that the complementary valve of the same pair is properly conducting electric current.
Electric power converters are used for interconnecting an electric power source and electric load circuits in applications where the loads utilize electrical energy of different form, frequency, and/or magnitude than supplied by the source. In many practical applications, the converter is used to derive polyphase alternating voltage of variable amplitude and frequency from a source of unipolarity voltage of relatively constant magnitude. Such a converter is commonly known and hereinafter called an "inverter." A typical inverter has relatively positive and negative direct current (d-c) conductors connected to the voltage source, two or three output terminals connected to a single-phase or polyphase alternating current (a-c) load circuit, and means for connecting each output terminal to both of the d-c conductors. The latter means comprises multiple pairs of alternately conducting controllable electric valves, each pair being serially connected between the d-c conductors, and the node or juncture of each pair being coupled to the corresponding output terminal. Associated control means is operative periodically to apply alternative turn-on and turn-off signals to the control electrodes of the respective valves in each pair of valves so that the valves switch between on and off states in a manner that converts the unipolarity voltage across the d-c conductors into alternating voltage of unipolarity voltage across the d-c conductors into alternating voltage of desired fundamental amplitude and frequency at the output terminals of the inverter. The present invention is particularly useful in conjunction with an inverter that is part of the electrical propulsion system on board a traction vehicle, such as a rapid transit car, where the source of d-c power comprises a third rail to which the system is connected via a sliding current collector on the vehicle and the a-c load circuit is a three-phase induction motor (or more than one such motor) the rotor of which is drivingly connected by suitable gearing to an axle of the vehicle.
Controlled turn-on and turn-off devices, such as gate turnoff (GTO) thyristors and power transistors, are often used for the inverter valves. The presently preferred devices are GTO thyristors. Such a valve is a multilayer semiconductor designed to freely conduct "forward" anode current (i.e., current flowing into its anode and out of its cathode) when its control electrode (gate) is triggered by a suitable turn-on or firing signal. A GTO thyristor is distinguished from a conventional thyristor by its ability to interrupt or block forward anode current if a voltage of relatively negative polarity and appropriate magnitude and duration is applied across its gatecathode junction. Such voltage is negative in the sense that the electrical potential of the gate is negative with respect to the cathode. It causes current to flow in a reverse direction in the thyristor's gate. In other words, to turn off a GTO thyristor, current is drained from the gate. Hereinafter such current is referred to as either "negative gate current" or the "turn-off signal."
In normal operation, the anode current-blocking or turnoff process of a GTO thyristor can be initiated at any time without waiting for a natural or externally forced zero crossing of the anode current. The turnoff process requires a finite period of time. During this process the negative gate current rapidly rises to a high peak that depends on the magnitude of anode current to be interrupted and then subsides as the thyristor recovers its ability to withstand off-state anode voltage. Once a turnoff process is successfully completed, the resistance of the gate-cathode junction is very high and limits negative gate current to a trivial magnitude.
To protect the inverter valves, a reactor is connected in series with each valve to limit the rate of change of current with respect to time (di/dt) when turning on and a capacitive snubber is connected in parallel with each valve to limit the rate of change of voltage with respect to time when turning off. In normal inverter operation, the complementary valves of each pair are so controlled that they will conduct load current alternately. That is, either the valve whose anode is connected to the positive d-c conductor is turned on to conduct current in one direction from the source to the load, or the opposite valve, whose cathode is connected to the negative d-c conductor, is turned on to conduct current in the reverse direction from the load to the source. If both valves were conducting simultaneously, they would establish an undesirable short circuit between the two d-c conductors.
To avoid simultaneous conduction of complementary valves, it is common practice to delay the turn-on signal for each one of the valves until a preset certain period of time has elapsed following the application of a turn-off signal to the opposite valve. This delay period is somewhat longer than the maximum time required for the valve to complete its normal turn-off process after receiving a turn-off signal. For a more positive indication that the previously-conducting valve has in fact turned off, its gate voltage can be monitored and compared with a predetermined reference value in the manner disclosed and claimed in U.S. Pat. No. 4,641,231--Walker and Lezan granted on Feb. 3, 1987. Interlocking circuits inhibit the application of a turn-on signal to each valve unless the monitored voltage of the opposite valve is more negative than the aforesaid reference value.
The referenced patent of Walker and Lezan discloses another method wherein the voltage across the main electrodes of a valve and the direction of load current are used to determine when the valve has turned off. Once its anode-to-cathode voltage exceeds a predetermined positive threshold magnitude at the end of a load current conducting interval, the valve is known to be turned off.
The above-mentioned delay period and interlocking circuits are usually effective to prevent simultaneous conduction of complementary valves under normal conditions. However, an inadvertent malfunction or fault condition in one of the valves or in the associated control or power circuits can cause an undesirable "shoot through," i.e., the simultaneous conduction of both valves. In this abnormal event the simultaneously conducting valves provide a short circuit between the positive and negative d-c conductors of the inverter, and current will rise very rapidly to a dangerously high magnitude. Once a shoot through occurs, the sooner it is detected the better.
One of the known characteristics of a GTO thyristor is that the maximum current it can successfully turn off is relatively limited. Typically a power-rated GTO thyristor has a turn-off limit less than approximately 300% of the maximum steady state rms load current rating of the thyristor. A GTO thyristor is very likely to be destroyed if an attempt were made to switch it from turned-on to turned-off states while it is conducting anode current higher than this limit. However, so long as such turn-off is not attempted, a GTO thyristor can safely conduct current having a magnitude much higher than its turn-off limit for a short length of time. In other words, a GTO thyristor is capable of conducting an occasional surge of current having a peak magnitude significantly greater than the maximum magnitude of current it can interrupt.
It is a known practice to provide overcurrent protective means for applying a turn-off signal to the gate of a turned-on GTO thyristor in response to anode current increasing above a pre-set magnitude that is higher than normal but lower than the thyristor's turn-off limit. If the rate of current increase is not too fast or the maximum current magnitude is not higher than the turn-off limit, the thyristor will have enough time, after an overcurrent condition is detected, to successfully complete its turn-off process before current can attain the turn-off limit. This would usually be true if the abnormal current rise were due to a fault in the electric load circuit, in which case the inductances in the load and interconnecting cables would tend to limit the rate of rise. However, it would probably not be true if the abnormally high current were the result of a shoot through. In the latter case, there is a steeply rising surge of current (e.g., 200 amps per microsecond) discharging the voltage-smoothing capacitor that is conventionally connected across the output of the power source to which the d-c conductors of the inverter are directly connected, and this current can rise above the turn-off limit of each GTO thyristor in a shorter time than required to complete the turn-off process after the abnormally high current is detected.